1. Field of the Invention
The present invention is a method for producing alumina trihydrate with a low silica content from Gibbsite-containing bauxite of low reactive silica content. The present method comprises two desilication steps which are effective in removing silica impurities: one step before extraction of alumina from the bauxite, i.e., a pre-desilication, and an additional desilication step following alumina extraction, i.e., a post-desilication.
2. Description of the Background Art
The Bayer process, widely described in the specialized literature, is the most important method used in the production of alumina. Alumina produced by the Bayer process can be used in the hydrate state, as transition alumina, calcined alumina, sintered or melted alumina, and can also be transformed into aluminum by igneous electrolysis.
In the Bayer process, bauxite is digested at an elevated temperature with an aqueous solution of sodium hydroxide with a concentration sufficient to solubilize the alumina to produce a supersaturated solution of sodium aluminate. After separation of the solid phase comprising undigested ore (red mud), the supersaturated solution of sodium aluminate is seeded with particles of alumina trihydrate in order to induce precipitation of alumina as alumina trihydrate.
The sodium aluminate liquor remaining after removing the precipitated alumina trihydrate, now poor in alumina, is known as decomposed liquor, spent liquor or depleted liquor. This decomposed liquor is then concentrated and recharged with sodium hydroxide to provide a digestion liquor which can be recycled in the digestion step.
It is well known to those of ordinary skill in the art that the conditions of the alkaline treatment of the bauxites in the Bayer process must be modified according to the degree of hydration and the crystallographic structure of the alumina as well as the nature and the content of the impurities found in the bauxite such as silica, iron oxides, humic materials, etc. Accordingly, bauxites containing alumina in the monohydrate stage (bohemite, diaspore) are treated at a temperature higher than 200.degree. C., generally between 220.degree. C. and 300.degree. C. Bauxites containing alumina in the trihydrate stage (hydrargillite, also called gibbsite) are digested at temperatures lower than 200.degree. C., and even at atmospheric pressure, which is easier to implement and results in important savings in operating costs by eliminating the need for autoclaves and pressure reactors.
Generally, the extraction yields of soluble alumina are higher than 95% and the purity of the resulting supersaturated sodium aluminate liquor, which influences the purity of the subsequently precipitated alumina trihydrate, is satisfactory if one is careful to limit the contents of these impurities by selective purification steps. The difficulty of these purifications, particularly int he case of silica, depends on the mineralogical forms of the impurities present.
Silica can be present in bauxite in several mineralogical forms that are not equally soluble in the sodium hydroxide solution. Some mineralogical forms of which Kaolin (Al.sub.2 O.sub.3,2SiO.sub.2,2H.sub.2 O) is the most common are solubilized along with the alumina trihydrate.
The term "reactive silica" is commonly applied to that fraction of the silica present in the bauxite in one of these forms, counted as SiO.sub.2, and usually accounts from 0.5 to 7% of the dry bauxite weight. In the presence of sodium hydroxide liquor, the reactive silica is first solubilized and then re-precipitated as sodium silico-aluminate.
The concentration of soluble silica in the sodium hydroxide liquor is determined by the solubility equilibrium of the sodium silico-aluminate after a very long time. During the industrial treatment of a bauxite containing alumina trihydrate, it is rare that the solubility equilibrium of the sodium silico-aluminate be reached. Usually, the silica concentration in the sodium liquor is greater, and even much greater, than the solubility equilibrium of the sodium silico-aluminate. This concentration is linked, consequently, to the solubility equilibrium of the sodium silico-aluminate and at the same time to its kinetic precipitation. This kinetic precipitation is slowed when the bauxite contains low amounts of reactive silica, because the precipitation of the sodium silico-aluminate is favored by the presence of reaction product.
In the Bayer cycle, the concentration of soluble silica in the alumina-enriched liquor after digestion of the bauxite is an important parameter because it determines that of the recycled digestion liquor as well as the level of silica impurity in the alumina product. It is therefore desirable to combine with the alumina extraction, a process known as "desilication" of the sodium aluminate liquor in order to reduce the silica concentration in this liquor, and therefore the level of the silica impurity in the alumina product. In order to allow the formation of insoluble sodium silico-aluminates, whose kinetic precipitation is relatively slow, desilication times of several hours are necessary, usually not exceeding ten hours, however.
This desilication process can be done during the digestion of the bauxite but preferentially during a distinct operation either preceding or following the digestion. See U.S. Pat. No. 4,426,363, FR 1506516 and U.S. Pat. No. 3,413,087, all incorporated herein by reference.
These desilication processes all place crushed bauxite in contact with all or part of the decomposed Bayer liquor, with a Na.sub.2 O concentration between 190 and 200 g/liter and at temperatures between 80.degree. C. and 200.degree. C., according to the nature of the bauxite to be treated. They usually afford satisfactory desilication performances with bauxite comprising less than 3% reactive silica based on the weight of dry bauxite.
On the other hand, using prior art processes for the desilication of alumina trihydrate containing bauxite in which the reactive silica content is less than 3% and often between 0.5 to 1.5%, requires a desilication time of at least 30 hours, or three times the usual desilication time, either before or after digestion of the bauxite. This long desilication period is necessary to reduce the concentration of soluble silica in the supersaturated sodium aluminate solution so that the ratio of soluble SiO.sub.2 /Na.sub.2 O is less than 0.90%, preferably less than 0.70%.
Under such conditions, the advantages expected from the digestion process occurring at atmospheric pressure are eliminated by the notable reduction of treatment capacity which can only be compensated by scaling up the process.
We note that the methods of EP 0203873 and U.S. Pat. No. 4,650,653, both incorporated herein by reference, of increasing the rate of the kinetic precipitation of sodium silico-aluminate by lowering the sodium hydroxide concentration to less than 120 g/liter in of the solution decomposed liquor used in the pre-desilication step, cannot be applied effectively in the present case because they are contrary to the desired goals herein including:
Preserving the existing production capacity and avoiding any significant increase of the time spent in the reactor and also any volume increase by dilution of the products circulating in the production line. PA1 Obtaining a productivity of at least 70 kg Al.sub.2 O.sub.3 per m.sup.3 of supersaturated sodium aluminate liquor. PA1 Obtaining an extraction yield of the potential soluble alumina of at least 95%, which is similar to the yields usually obtained with the other types of bauxite. This implies not only a very complete digestion of the ore but also the prevention of any retrogradation that could lead to a significant decrease in yield of alumina, which can be 5 to 10% even 20% of the final product. PA1 Limiting the soluble silica content in the supersaturated liquor before decantation and decomposition, measured by the weight ratio of soluble SiO.sub.2 /Na.sub.2 O, to less than 0.90%, and preferably less than 0.70%, to guarantee a silica content in the precipitation alumina trihydrate less than 100 ppm.
This productivity, P, is the product of the concentration, C, of sodium hydroxide in said liquor and the variation, .DELTA.Rp, of the concentration ratio, Rp, defined as soluble Al.sub.2 O.sub.3 (g/l)/Na.sub.2 O(g/l), between the beginning and the end of the decomposition. It is advantageous to keep the sodium hydroxide concentration in the digestion as high as possible, since this concentration also determines the maximum value of Rp before decomposition and therefore the magnitude of .DELTA.Rp. In fact, during the dilution and the decantation of the suspension obtained following digestion of the bauxite, the risks of precipitation by spontaneous nucleation of part of the alumina trihydrate, known as retrogradation, are greatest when the sodium hydroxide concentration is low. In the present case, the maximum Rp before decomposition cannot be greater than 1.05 which practically limits the productivity to 70 kg Al.sub.2 O.sub.3 /m.sup.3 for a final Rp preferably between 0.5 and 0.7.